1. Field of Invention
The invention relates generally to determining the condition of tools and workpieces.
2. Discussion of Related Art
My U.S. Pat. Nos. 4,613,812 ('812) and 5,090,847 ('847) are hereby incorporated by reference. The '812 patent describes a microwave detection system for determining the continuity or discontinuity of a target, e.g., a drill bit during a machine-tool drilling operation. The '847 patent describe a pressure foot useful in a microwave detection system for improving detection of the presence of very fine drill bits, such as those used in fabricating microelectric circuits.
In the last decade, the machine tool and Printed Circuit Board (PCB) industry have its technical demands. The introduction of surface mount technology has promoted the use of smaller diameter drills. The holes are used not only to mount components but also to provide conductive paths from one level to another. The holes are often "elevator shafts" for connecting a circuit from one level to another in a multi-level printed circuit board. Therefore the size of the hole need only provide a path big enough for the required flow of electrons. The size limits on holes to be filled with "reflow solder" are a constraint that prevents the size of the holes from going to almost zero. In 1986, a hole diameter of 0.0139" (#80 drill) was considered very small, and less than 10% of the industry was regularly drilling holes that size. Today, 0.0139" is not just a frequently drilled diameter: if a supplier lacks that capability, then that lack will limit the supplier's customer base. The diameter that is now considered small is 0.0059", the diameter of a #97 drill, and some companies are trying to establish a capability for drilling a 0.003" diameter hole. To put this in perspective, an average human hair is 0.006" in diameter.
The advent of smaller circuit boards and microelectronics has made it necessary to use multilayer circuit boards to complete the conductive paths needed to make the circuits work. This in turn made it important to have some holes only drilled to a certain depth when these complex, multilayer boards are fabricated. Below that depth may be a conductive path that should not be connected to the path created through the hole. If the hole is drilled too deeply, solder in the hole will electrically connect all the layers through which the hole is drilled. Depth control can be most accurately maintained by indicating to the computer controlling the drill when the drill bit has actually touched the top of the PCB stack.
Currently there are techniques that use laser locator beams or mechanical "touch plates" to determine the location of the tip of a tool. This is done at a setup station at the edge of the drilling machine where the tool location relative to the pressure foot or a scale on the spindle z-axis is calibrated. The laser locator systems do work fairly well but are limited in their ability to detect the location of drill bits having very small diameters. The mechanical method has the significant disadvantage of occasionally damaging the tip of the tool. A small chip or other deformation of a cutting edge greatly affects the life of a tool and increases the potential for premature tool breakage.
Furthermore, the surface of a large PCB is not completely flat, and may warp or have surface variations or discontinuities. If the warp or variation is more than the depth of one of the several layers of the board, then the depth of some of the holes drilled may be either too shallow to make needed contacts to the layers, or too deep and thus make unwanted contact to a layer beneath the desired layer.
A further prior art shortcoming is the current wear-control policy of the PCB industry, which uses a given tool diameter for a preset number of holes referred to as "hits". In most cases this number of hits is low compared to what the tool can withstand before it is likely to break. Furthermore, the resharpening of a drill is a standard practice in the industry. The resharpened or repainted tools by nature have a different and less predictable life span. Therefore the need to determine when a tool is chipped or has some other condition that leads to a premature breakage would obviously be of great value to the industry. In addition, the ability to track the wear factor of a repainted tool would be extremely valuable since the serviceable life of a resharpened tool is not as exact as that of new tools.
The magnitude of the problem of tool breakage, excessive tool wear, and chipping or other damage, may be appreciated from the way complex printed circuit boards are drilled. A complex board may have thousands of holes drilled into it, and a computer controlled machine may take up to 4 hours to drill all the holes. If a drill breaks, some facilities discard an entire board that may have been affected by the drill's defect, to assure high quality. A worn drill or damaged drill will not produce acceptable quality holes and it is important to detect, or better yet predict, a defect in the drill. Thus, it is important to know when a tool will break.
In contrast with the Printed Circuit Board (PCB) industry, the machine tool (MT) industry has both different and similar conditions and needs. The MT industry currently has several options for detecting drill bit breakage. One of the options is to use a mechanical wand that rotates in to touch the tool after each drill stroke. Another option is a mechanical vibration monitor that "feels" if the machine vibrates differently during the drill stroke. A third option is a load monitor to track the current that the spindle motor uses to turn a tool attached to the spindle. If that current suddenly changes, then the system interprets that change as indicating a broken too. The manufacturer has maintained that this load monitor can also detect tool wear. This may be true, but only of the largest tool.
The MT industry still has a need to find a better way to detect the broken tools because each of the aforementioned methods has a weakness. The mechanical wand is subject to breakage and misalignment, and the contact of a mechanical wand with a tool can damage the tool. The vibration differences a machine experiences can have causes other than tool breakage; thus, the vibration monitors not a fully reliable method. A load monitor can interpret other current changes, such as line spikes and electrical noise, as tool information and this makes load monitors not very reliable.
The ability to reliably determine the wear status of a tool would greatly extend the useful life of a tool, yet make it possible to replace the tool before breakage occurs.